pthread_attr_getguardsize(3) - NetBSD Manual Pages

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PTHREAD_ATTR_GETGUAR... NetBSD Library Functions ManualPTHREAD_ATTR_GETGUAR...


NAME

     pthread_attr_getguardsize, pthread_attr_setguardsize -- get and set
     thread guard size


LIBRARY

     POSIX Threads Library (libpthread, -lpthread)


SYNOPSIS

     #include <pthread.h>

     int
     pthread_attr_getguardsize(const pthread_attr_t * restrict attr,
         size_t * restrict guardsize);

     int
     pthread_attr_setguardsize(pthread_attr_t *attr, size_t guardsize);


DESCRIPTION

     The pthread_attr_getguardsize() and pthread_attr_setguardsize() functions
     get and set guardsize in the attr object.  If guardsize is larger than 0,
     the system reserves an additional region of guarded memory of at least
     guardsize bytes at the end of the thread's stack for each new thread cre-
     ated by using attr.

     The guarded area is understood to be pages of memory that are protected
     from read and write access.  While the guarded area should be rounded by
     the system page size, the actual default size is implementation-defined.
     In NetBSD the default guardsize is given by the vm.thread_guard_size
     sysctl(7).

     The rationale behind guardsize is two-fold:

       1.   On the one hand, it provides protection against overflow of the
            stack pointer.  If there is a guard area and a thread overflows
            its stack pointer into this extra memory area, it should receive a
            SIGSEGV signal or experience other comparable fatal error condi-
            tion.  Note that if a thread allocates large data structures on
            stack, it may be necessary to raise the default guardsize in order
            to detect stack overflows.

       2.   On the other hand, the overflow protection may waste system
            resources if an application that creates a large number of threads
            knows that it will never overflow the stack.  In this case it is
            possible to set guardsize to 0.

     If pthread_attr_setstack(3) or pthread_attr_setstackaddr(3) is used to
     set the stack address attribute in attr, the guard size attribute is
     ignored and no guard area will be allocated; it is the responsibility of
     the application to handle the overflow conditions.


RETURN VALUES

     If successful, both functions return 0.  Otherwise, an error number is
     returned to indicate the error.


ERRORS

     No errors are defined for pthread_attr_getguardsize().

     The pthread_attr_setguardsize() may fail if:

     [ENOMEM]           There was insufficient memory.


SEE ALSO

     pthread_attr(3), pthread_attr_setstack(3), sysconf(3)


STANDARDS

     Both functions conform to IEEE Std 1003.1-2008 (``POSIX.1'').


BUGS

     Older versions of NetBSD, prior to 10.0, 9.4, and 8.3, incorrectly adjust
     the stack address by the guard size in threads configured with
     pthread_attr_setstack(3), instead of ignoring the guard size in that case
     as POSIX prescribes (see PR lib/57721:
           https://gnats.NetBSD.org/57721).

     Even if you didn't set a nonzero guard size with
     pthread_attr_setguardsize(), the system will choose a nonzero default
     guard size.

     To work around this in applications that run on older and newer versions
     of NetBSD, as well as on other operating systems, you can safely set the
     guard size to zero:
           pthread_attr_setguardsize(&attr, 0);

NetBSD 10.1                      July 2, 2017                      NetBSD 10.1